1. Field of the Invention
The invention relates to a light emitting device comprising a light emitting chip attached on a heat conductive layer by means of an undercoating.
2. Description of the Related Art
One problem of the existing light emitting devices is the high thermal resistance from the device to the ambient air. This is due to the stack-up of the device like, for example, the use of a printed circuit board or a lead frame.
Since heat conduction of the devices is directly related to efficiency and life time of the light emitting devices, it is desirable to produce devices with good heat conduction.
In addition, the complexity of light emitting devices comprising a printed circuit board is very high so reducing this is an additional goal of some embodiments of this invention.
Most of the light emitting devices comprise an LED chip which is attached to a leadframe. The leadframe is usually metal and is encased in a plastic or ceramic package. This package is normally made of a plastic or ceramic material. These materials have a low thermal conductivity. In addition, the packaged chip can be attached to the board by a glue or solder and the board is attached to the heat sink by a glue, thermal interface like a paste, grease or an epoxid or another adhesive. This glue and the adhesive materials also have a low thermal conductivity. Furthermore, the material of the printed circuit board which is typically an FR4-material (woven glass and epoxy) or a CEM-material (cotton paper and epoxy or woven glass and epoxy), both have a low thermal conductivity.
The heat which is produced during operating the light emitting device by the chip has to pass the package into the board substrate and the thermal interface material or air between the board and the heat sink until it reaches the heat sink and can be dissipated to the ambient.